# Friction acting on a wheel that is initially at rest

Hello everyone,

I have a project of mine where I need to find out the force that acts on a wheel to start it rolling and keep it rolling so I can then select the correct motor and gear box.

I have not had to do this kind of physics for years and I am having trouble with this problem:

If a wheel is initially at rest then the standard static friction equation would apply (F = u*Normal) initially to find the force to get the wheel moving and then the rolling friction force equation would be used to work out the force required to keep it going?

or

Because it is a wheel the initial and ongoing friction would be determined by using the rolling static friction equation in both cases?

I am really confused here and would really appreciate the help.

Thanks. :)

Doc Al
Mentor
If a wheel is initially at rest then the standard static friction equation would apply (F = u*Normal) initially to find the force to get the wheel moving and then the rolling friction force equation would be used to work out the force required to keep it going?
To get the wheel rolling requires static friction (assuming no slipping). But μN gives the maximum amount of friction available to you; the actual friction required depends on the acceleration you are trying to impart. (The actual static friction could be anything from 0 to the maximum value.)

ok thanks, so F=uN is max force.

what equation do I use if I have an acceleration in mind to find the force?

Doc Al
Mentor
ok thanks, so F=uN is max force.
Right.
what equation do I use if I have an acceleration in mind to find the force?
Newton's 2nd law for rotation and translation.

F=m*a......
now I feel like an idiot, but I am glad I asked.

Doc Al
Mentor
F=m*a......
That will do for translation; for rotation, use Torque = I*alpha.

rcgldr
Homework Helper
Doc Al
Mentor
The opposing force is due to rolling resistance, not static friction.
To accelerate the wheel from rest, static friction is required.